This invention relates to a permanent magnet and the method for producing the same, more particularly to a permanent magnet which has high maximum energy product and good corrosion-resistance.
Since 1970s, rare earth type magnets have been widely used for motors, radios, etc. because their maximum energy product is 3 to 10 times of that of conventional magnets, which are made of, for example, Al-Ni-Co, Ba ferrite. Two types of rare earth permanent magnets, Sm-Co magnets and Nd-Fe-B magnets, have been proposed. Sm-Co magnet has good corrosion-resistance although its maximum energy product is comparatively lower (about 16 to 30 MGOe). Nd-Fe-B magnet has a high maximum energy product (over 25 MGOe), but it has a poor corrosion-resistance. In these cases, a rare earth permanent magnet cannot possess both a high maximum energy product and good corrosion-resistance.
In addition, because the rare earth elements are highly reactive, they are therefore liable to react with oxygen and nitrogen in the air, resulting in deterioration of the magnetism of the permanent magnet produced therefrom. Therefore, conventional rare earth magnets must be sintered in an inert gas, such as argon and helium during the manufacturing process. However, because the inert gas is expensive, the manufacturing cost of such permanent magnets is high. U.S. Pat. No. 3970484 discloses a method for producing a Sm-Co magnet including sintering the Sm-Co magnet in hydrogen atmosphere in order to reduce the manufacturing cost. However, it is very dangerous to use hydrogen gas at a high temperature during the sintering process.